As is well known, motorcycles are generally built upon and around a frame assembly that is made up of a plurality of individual tubes welded together. The typical frame construction includes a head pipe at a front end of the frame, which desirably supports the front wheel via a front fork assembly which is steerable by an attached handle bar(s). A main tube, or tubes, typically extend generally rearwardly from the head pipe and may bend downward over a rear portion of an engine that is positioned generally below the main tube.
In addition, one or more down tubes are typically affixed at their forward ends to the head pipe at a location lower than the main tubes. The down tubes extend downwardly and rearwardly from the head pipe. The rear ends of the main tube and the down tubes generally are joined, frequently by a pair of seat rails and bracket assemblies. The bracket assemblies typically provides support for the rear wheel suspension swing arm assembly. Although the arrangement of the individual frame members may vary, the above-described construction is generally the type utilized for motorcycles for both on and off-road motorcycles.
In the case of lateral loads applied to a motorcycle, rigidity is desired to improve stability during cornering at high speeds or over rough terrain. In the case of the vertical loads, the motorcycle must absorb certain vertical shock loads applied to the frame. If the frame is too rigid, the shock loads are largely transferred to the rider. If the frame is not rigid enough, the frame may elastically deform or break.
A certain frame for motorcycles having a single, rectangular cross-section main tube are disclosed, for example, in Itoh et al, U.S. Pat. No. 5,845,728. This frame includes a down tube provided for supporting a forward part of the engine extending downward from the head tube. It is clear from the description and the drawings of this reference that the down tube does not directly connect with another part of the frame, but does connect directly to the engine for support thereof and also that the single main tube of the frame is rectangular or elliptic in cross-section, or in other words, transversely elongate in cross-section.